An ortho-mode transducer ("OMT") is a three-port waveguide device which supports signals having two orthogonal modes. For purposes of discussion, the two orthogonal signal modes will be designated as H and V linear polarities. A conventional OMT is shown in FIGS. 1A-1G. The common port (port 1) is a circular, square, or similar type of waveguide portion which supports both H and V polarization signals. The through port (port 2) is a waveguide portion aligned with the common port waveguide and which supports only V polarized signals. Port 3, the side port, is a waveguide which splits off from the common and through port waveguides and supports only H polarized signals.
OMTs are often used in reflector antenna systems to separate H and V polarized signals. The combined signal is received, i.e., as focused energy from a parabolic reflector, and applied to the common port of the OMT through a feedhorn. The received V and H polarized signals are separated and output via the through and side ports, respectively. OMTs are also used in applications when the antenna system transmits H polarized signals and receives V polarized signals. For this application, the H polarized output signal is transmitted from a power amplifier module into the through port of the OMT, where it is directed into the common port and output into the feed horn and the reflector. V polarized signals are funneled by the feed horn into the common port of the OMT, where it is directed into the side port and into a receiver module (containing, for example, a filter, amplifier, down converter, etc.). For receive only antenna systems or transmit/receive antenna systems the orthogonal through and side ports can be designed to cover the same, distinctly different or overlapping frequency bands.
Good port to port isolation is critical to applications that transmit from the V port and receive on the H port because the power transmitted from the V port toward a distant satellite or terrestrial hub is very high in comparison to the low power received at the H port. In conventional OMT designs, signal separation and isolation between the through and side ports is achieved by providing a septum or reduction in height in the body of the OMT near the junction between the common and through waveguide portions. The septum or height reduction redirects H polarity signals from the common port into the side port, while allowing V polarity signals from the common port to continue into the through port. The arrangement also works in reverse, channeling both V polarity signals entering the through port and H polarity signals entering the side port into the common port. This mechanism, together with the orthogonal orientation of the through and side ports, provides relatively good isolation between through and side ports. In other words it allows only a small amount of the energy of H polarity signals to enter the through port and very little V polarity signal energy to enter the side port.
Although conventional OMT designs offer good port to port isolation and functionality, the structure is asymmetric with respect to the common port because the through, or V port is aligned with the common port, while the side, or H port is orthogonal to the common port. This asymmetry can degrade port to port isolation. It can also result in degraded cross polarity (x-pol) rejection, i.e., the V port's rejection of the H polarity coming from the common port, and the H port's rejection of V polarity coming from the common port.
Furthermore, because all three ports lie in the same plane, and because the V port is axially aligned with the common port, the feed antenna connected to the common port will lie along the same axis as any transmit or receive elements connected to the V port. This results in a bulky assembly which is unsuitable for many applications.
Accordingly, it is an object of the invention to provide an OMT wherein both the H and V ports are in the same plane and are orthogonal to the common port.
It is a further object of the invention to provide an OMT with improved cross polarity rejection.
Yet another object of the invention is to provide an OMT which may be inexpensively fabricated as two planar elements joined together, which elements contain the necessary filters, waveguides, etc. for integrating the OMT and with a transmit package and/or a receive package.